Charge state monitoring circuits and reserve indicator circuits are known for batteries and storage batteries, wherein the charge state of the battery in question, said battery being connectable via a switch to an electrical consumer, is monitored by a reference element, whereby the falling of the battery or storage battery voltage below the voltage level presettable by a reference element triggers an acoustic and/or optical device, indicating to the user of the battery-powered electrical consumer that the charge state of the battery or storage battery necessitates replacement or recharging in the near future. The critical voltage value is set so that the amount of charge still in the battery or storage battery when the indicating device is activated permits operation of the electrical consumer for a given space of time. Such acoustic and/or optical indicating devices are provided especially for example in battery or storage battery powered electrical appliances such as electric shavers, electric toothbrushes, or flash units. A disadvantage of the known circuits is that when the voltage falls below the set reference value, a continuous energy drain from the battery or storage battery occurs to power the indicating device, and that the indicating device is activated only when the appliance switch is turned on, so that the user of the electrical device is frequently not aware of the warning signal when using the device or fails to hear an acoustic signal while the device is operating.
Other known battery testing circuits can comprise a battery test button in addition to the appliance switch which, when actuated, provides an indication of the charge state of the battery, in which the no-load voltage is measured, possibly with the aid of a reference element. One disadvantage of these known battery test circuits is that an additional test button must be provided, necessitating additional manufacturing cost and not always actuated during operation, so that if the button is not actuated and a critical charge state of the battery occurs, the battery can be discharged excessively without the user noticing the charge state of the battery. This also applies to designs in which a special battery test position of the operating mode switch is provided instead of an additional battery test button.
A battery-condition tester for a vehicle having a storage battery and a charger connected with the battery is illustrated and described in U.S. Pat. No. 4,413,234 wherein the tester comprises a pair of light-emitting indicators for indicating a low battery condition as well as a normal battery condition. The indicators are selectively energized through a current control means comprising a reference voltage means and a switch which turns on said light-emitting indicators whenever the charge state of said storage battery drops below a predetermined value or is within a predetermined range. However, the known arrangement is such that at least one indicator is illuminated at all times and thus consumes energy from said storage battery. Furthermore, the known tester fails to provide an indication of the battery condition depending upon the specific load.
Another arrangement is illustrated and described in U.S. Pat. No. 4,242,672 wherein a circuit is described which provides an audable alarm to be given when a battery voltage drops below a certain level indicative of the need to replace the battery. This known arrangement includes a three position control switch which establishes off, on and test modes of operation whereby in the test mode of operation a low voltage monitor serves to monitor the battery voltage and to call an audable alarm when the battery voltage drops below a predetermined critical level. The low voltage monitor includes a zener diode and a resistor connected in series across the battery such that so long as the battery voltage remains above a critical level the zener diode is conductive. Whenever the battery voltage passes through a critical level, the zener diode become less conductive such that the base of a transistor is triggered and by activation of this transistor the audable alarm is activated. The known circuit needs an additional position of the switch in order to monitor the charge state of the battery.
A monitoring means for a combustion engine electric storage battery means is shown and described in U.S. Pat. No. 4,488,521 having an electrical system monitor for sensing the actual voltage across the storage battery and for monitoring the current through the storage battery. The known device comprises a plurality of C-MOS type integrated circuit gates and a plurality of resistance means for monitoring the charge state of the battery and for providing an output signal indicating that the monitored voltage is between two points of reference.